Reflective film

ABSTRACT

Provided is a reflection film for a backlight optical system as a constituent of a display having a high quality by preventing damage on a light guiding plate, especially a polyolefin-based light guiding plate, with intrinsic performances of the reflection film such as an optical characteristic, a blocking property, tractability and the like sustained. A reflection film used in a backlight optical system consisting of a light source, a light guiding plate and a reflection film, wherein a resin layer containing particles with elasticity is provided on a surface, in contact with the light guiding plate, of the reflection film. The reflection film contains particles with elasticity therein. It is preferable that the particles with elasticity each have a cushioning property and elasticity and a rubber hardness (JIS K6253) of 50 or less. Specifically, the particles with elasticity each are preferably made of silicone, crosslinked polyacrylate ester, polyurethane or the like. The diameters of the particles with elasticity are preferably in the range of from 1 to 60 μm.

TECHNICAL FIELD

The invention relates to a reflection film. More particularly, theinvention relates to a reflection film with high homogeneity used in abacklight optical system for a liquid crystal display.

BACKGROUND ART

A liquid crystal display is rapidly enlarging its application fields asa display means for IT-related equipments such as a computer, atelevision set, a mobile, communication equipment and the like.Especially, reduction in weight, downsizing, an improved display qualityand such have been strongly requested in a notebook personal computer, aportable telephone, a mobile and the like from the viewpoint ofportability and convenience. Along with such a trend, reduction inweight, downsizing and, in addition, toughness have also beenincreasingly desired in a backlight unit, which is a component of suchIT-equipment.

A sidelight type backlight unit is basically, as shown in FIG. 3, anoptical system consisting of a light source 5, a light guiding plate 4and a reflection film 1. The light guiding plate has been conventionallymade of an acrylic resin plate from the viewpoint of an opticalperformance and formability, whereas a light guiding plate made of apolyolefin-based resin has been employed so as to make it to be adaptedfor various requests such as downsizing, reduction in weight, toughness,heat resistance, moisture resistance, low warping, low bending, improveddisplay quality and the like. The reflection film provided on the backsurface of the light guiding plate has been made of a polyester-basedresin or a polyolefin-based resin.

A polyolefin-based resin light guiding plate is preferably made of, forexample, Zeonor resin manufactured by Nippon Zeon Co., Ltd. Apolyolefin-based resin light guiding plate has a specific gravityconsiderably smaller than that of an acrylic plate, and thus, it iseffective for downsizing and reduction in weight.

A polyolefin-based resin light guiding plate, however, has had a problemof receiving pressure damage easily because of the presence of aninorganic additive included in a depressed or protruded portion on thesurface of the reflection film when constructing a backlight opticalsystem by combining the light guiding plate and the reflection film.When the light guiding plate receives pressure damages, bright spots anddark spots are produced on the plate at the location of pressure damage,thus, resulting in loss of a quality of a display.

It is an object of the present invention to provide a reflection filmfor a backlight optical system as a component of a display having a highquality free of damage on a light guiding plate caused by a property ofa surface of the reflection film or a physical quality of the film withintrinsic performances of the reflection film such as an opticalcharacteristic, a blocking property, tractability and the likesustained, especially, in a case where a polyolefin-based light guidingplate is employed.

DISCLOSURE OF THE INVENTION

The invention is directed to a reflection film wherein a resin layercontaining particles with elasticity is provided on a surface, incontact with a light guiding plate, of the reflection film.Alternatively, the invention is directed to a reflection film containingparticles with elasticity therein. In another words, the reflection filmaccording to the invention is a reflection film used in a backlightoptical system consisiting of a light source, a light guiding plate anda reflection film, and has a structure in which a resin layer containingparticles with elasticity is provided on the surface, in contact with alight guiding plate, of the reflection film. Alternatively, it is areflection film used in a backlight optical system consisiting of alight source, a light guiding plate and a reflection film and is areflection film containing particles with elasticity therein.

The light guiding plate is preferably made of a polyolefin-based resin.While what is generally used as a light guiding plate is an acrylicresin plate or a polyolefin-based resin plate, it is preferable toemploy a polyolefin-based resin plate in the invention from theviewpoint of downsizing and reduction in weight. It is preferable thatthe particles with elasticity (hereinafter also referred to as elasticparticles) each has a cushioning property and elasticity and a rubberhardness (JIS K6253) of 50 or less. Specifically, the particles withelasticity are each preferably made of at least one kind selected fromthe group consisting of silicone, crosslinked polyacrylate ester andpolyurethane. Particles made of the respective resins included in thegroup may be employed either in a single kind or in a mixture of two ormore kinds.

The elastic particles each are preferably spherical. This is becausespherical particles are preferably spread over a reflection film easily.Diameters of the elastic particles are preferably in the range of from 1to 60 μm and more preferably in the range of from 1 to 40 μm. Theaverage diameter of the elastic particles (hereinafter it means indiameter) is preferably in the range of from 5 to 20 μm.

The reflection film is generally made of a polyester-based resin or apolyolefin-based resin. In the invention, a film can be employed that isproduced by stretching a film made of a polyester-based resin or apolyolefin-based resin containing an inorganic filler such as calciumcarbonate, titanium oxide or the like to form a number of microvoidstherein and to thereby cause the stretched film to have a lightreflection function. Furthermore, a transparent film can be employedthat is made of a polyester-based resin or a polyolefin-based resin, oralternatively, a white film can be employed that is produced by addingtitanium oxide or the like into the transparent film. Moreover, therecan be employed a reflection film with a vapor deposited layer made ofsilver or aluminum thereon.

One of the features of the invention resides in providing a resin layercontaining elastic particles on the surface, in contact with a lightguiding plate, of a reflection film. Another feature of the inventionresides in a reflection film containing elastic particles being therein.With a resin layer containing elastic particles provided, the elasticparticles serves as a cushioning material between a light guiding plateand the reflection film when the plate and the film become in contactwith each other, thereby preventing the light guiding plate from beingdamaged. With a reflection film containing elastic particles therein, asimilar effect is obtained. Especially, in a case where a light guidingplate is made of a polyolefin-based resin, the effect is enhancedconsiderably.

The reflection film is made of a polyester-based resin or apolyolefin-based resin. This is because these resins are high in lighttransparency and are excellent in durability. As for the binder whichfixes particles to the film and which s at least one or more kindsselected from the group consisting of a polyester resin, an acrylicresin, a silicone acrylic resin, a fluororesin and a fluorine-containingacrylic resin; the resins to which a crosslinkable resin having ahardening function is added; and curable resins such as apolyurethane-based resin, an epoxy-based resin and the like.

The reflection film of the invention can be produced by a procedure inwhich the elastic particles are mixed with a binder in a solvent, andthe mixture is coated on the film and after the coating, a heattreatment is applied to the film to remove the solvent to thereby fixthe elastic particles onto the film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the construction of a backlight optical systemusing a reflection film of the invention.

FIG. 2 is a view showing the construction of a reflection film of theinvention.

FIG. 3 is a view illustrating a general backlight optical system.

BEST MODE FOR CARRYING OUT THE INVENTION

Description of the invention will be given based on embodiments. A basicconstruction of the invention is shown in FIG. 1 and an enlarged view ofthe reflection film is shown in FIG. 2. A backlight optical system ofthe invention basically includes a light source 5; a light guiding plate4; and a reflection film 1 having a resin layer 2 containing elasticparticles 3 therein. The resin layer 2 containing the elastic particles3 is provided on a surface, in contact with the light guiding plate 4,of the reflection film 1. The elastic particles 3 resides between thelight guiding plate 4 and the reflection film 1 to thereby absorb apressure therebetween to prevent the light guiding plate 4 fromreceiving a damage. Though not shown in the figure, the reflection filmcontaining elastic particles therein also shows the effect of preventingthe light guiding plate from being damaged.

A reflection film 1 is a film or a white film made of a polyester-basedresin or a polyolefin-based resin. The white film is fabricated in aprocedure in which a plastic resin is added with, for example, a pigmentsuch as titanium oxide, barium sulfate, calcium carbonate, aluminumhydroxide, magnesium carbonate, aluminum oxide or the like is added to aplastic resin so as to make it white before the plastic resin is formedinto a film or a sheet, and then formed into a film or a sheet.Alternatively, a reflection film can also be used that is fabricated ina procedure in which an inorganic filler such as calcium carbonate,titanium oxide or the like is mixed into a resin, followed by stretchingto form a number of microvoids therein. In order to enhance a reflectionefficiency, a reflection film can also be used that has a vapordeposited layer made of silver or aluminum thereon. The reflection filmmay be coated with a resin layer, thereon, that contains a pigment suchas titanium oxide, barium sulfate, calcium carbonate, aluminumhydroxide, magnesium carbonate, aluminum oxide or the like so as to makeit white.

While an acrylic resin plate or a polyolefin resin plate is used as thelight guiding plate 4, a polyolefin-based resin plate is preferablyemployed from the viewpoint of downsizing and reduction in weight of thebacklight optical system.

The elastic particles used in the invention are particles with acushioning property and an elastic property. The elastic particlespreferably have, as the standard of elasticity, a rubber hardness (JISK6253) of 50 or less, and more preferably have a rubber hardness (JISK6253) of 30 or less. As the specific examples of the elastic particles,there can be presented particles made of silicone, crosslinkedpolyacrylate ester, polyurethane or the like. Employed is at least onekind of the elastic particles presented above. The elastic particles maybe employed alone or in a mixture of two or more kinds.

Silicone elastic particles that can be preferably employed areencapsulated with a resin from the viewpoint of compatibility with, anadhesion property to and dispersibility in the binder resin.

No specific limitation is imposed on the shapes of the elasticparticles, but the shape of each elastic particle is preferablyspherical from the viewpoint of uniformity in depressions andprotrusions of the resin layer containing particles to be formed,dispersibility in a binder resin and the like. The sizes of theparticles are preferably in the range of 1 to 60 μm in diameter. Thesizes of the particles are more preferably 40 μm or less in diameter.The average particle diameter is preferably in the range of from 5 to 20μm. If the particle diameters are less than 1 μm, a blocking propertyand a cushioning property are hard to be obtained between the lightguiding plate 4 and the reflection film 1, while if the particlediameters exceed 60 μm, a thickness of a resin layer is necessary to belarge from the viewpoint of an adhesion property between the binderresin and the particles and retainability of the particles in the resinlayer (the particle is hard to fall off). With a thick resin layer beingadopted, a glossiness of the resin layer increases with the decrease inreflectance of the reflection film.

A binder resin is used in order to fix the elastic particles 3 to thereflection film 1, which is a substrate material. Employed as the binderresin is a resin good in resistance to light and transparency, such aspolyester resin, acrylic resin, silicone acrylic resin, fluororesin orthe like. Resins that can be added to a binder resin are a crosslinkableresin, such as an ultraviolet curable resin, an electron beam curableresin, a thermo-curable resin, an isocyanate-curable resin, anepoxy-curable resin and the like. The thickness of a supporter ispreferably in the rage of from 30 to 300 μm and more preferably in theorder of 50 to 200 μm from the viewpoint of tractability, reflectioncharacteristic and reduction in weight.

A resin described above and elastic particles are mixed in a solvent.Then, after the solution containing the elastic particles and the binderresin are adjusted to show a proper viscosity, and the solution iscoated on a reflection film 1. After the coating, the coat on thereflection film 1 is dried and heat treated to fix the elastic particlesto the predetermined plastic film. In addition, a screen printingmethod, a coating method or the like can be applied on the surface ofthe reflection film to form a resin layer containing elastic particlesthereon.

A reflection film containing elastic particles can be fabricated bymixing the elastic particles into a resin to extrude the mixture, forexample, through a T-die, to form a film-like sheet and to then stretchthe sheet when required.

The amount of elastic particles to be mixed may be 0.5 part by weightrelative to 100 parts by weight of a binder resin only for preventing adamage on a light guiding plate 4. The mixing ratio of elastic particlesis, however, preferably in the range of 0.8 part by weight to 200 partsby weight relative to 100 parts by weight of a binder resin from theviewpoint of the light reflection characteristic, the blocking property,tractability and the like of the reflection film 1. If the mixing ratioexceeds 200 parts by weight, the elastic particles tend to fall offeasily.

The reflection film thus fabricated is cut to obtain a piece having asize that matches the size of the light guiding plate, or alternatively,the reflection film thus fabricated is imparted with a print patternthereon for light adjustment, and thereafter, cut to obtain a piece,which is disposed on a back surface of the light guiding plate so as tobe in contact therewith. A reflection film is, in a case, used togetherwith a lamp reflector to form a single body by means of a bendingmethod, a score cutting method or a half cutting method.

A resin layer containing elastic particles preferably have also ablocking function between the light guiding plate and the reflectionfilm in addition to the purpose of preventing the damage to the lightguiding plate. For that purpose, it is not preferable to have anexcessively thick resin layer since it is undesirable to have all of theelastic particles buried in the resin layer. Since the elastic particlesfall off when a thin resin layer is adopted, a certain level of theresin layer's thickness is required. The thickness of the resin layer ispreferably in the order of one/fifth to four/fifths of the averageparticle diameter of elastic particles.

EXAMPLES 1 TO 6

Prepared was a white polyester film (produced by TORAY INDUSTRIES, INC,with a trade name of E60L) with a thickness of 188 μm containing calciumcarbonate. Then, various kinds of elastic particles (a rubber hardnessof 30) shown in Table 1 were mixed in a solvent consisting of toluene,methyl ethyl ketone and butyl acetate to form a solutions. Viscositiesand the like of the solution were adjusted, and the solution was appliedon the reflection surface of the polyester reflection film, and the coatthereon was dried and heat treated and subjected to aging to therebyform a resin layer containing elastic particles and obtain a reflectionfilm. Thus obtained film was cut into a piece having a proper size andwas combined with the polyolefin-based light guiding plate to form abacklight optical system.

A load of 10 kg was applied on the back surface of each of the backlightoptical system over an area of the order of 5 mm in diameter. After thebacklight optical system was left for 24 hours under the load, visualinspection was conducted on the backlight optical system to determinethe level of damage and light spots of the light guiding plate. Resultsof the inspection are shown in Table 1. Note that a column of addedamounts in Table 1 shows part or parts by weight of elastic particlesadded with respect to 100 parts by weight of a binder resin and symbolsin columns of damage and light spots on the light guiding plates areevaluated at three ratings, in which a mark □ is a level at whichneither damage nor a light spot is recognized on a display when a lightsource is turned on, a mark ∘ is a level at which a recess on theslightest scale can be recognized without a practical problem beingposed and a symbol x is a level at which damage or light spots can berecognized with a practical problem being posed. TABLE 1 damage onaverage diameter added amounts film thickness light light Exampleelastic particles (μm) (parts by weight) (μm) guiding plate spots 1silicone 10 0.8 5 ◯ □ particles (spherical) 10 10 ◯ □ 200 5 ◯ □ 10 ◯ □ 5◯ □ 10 ◯ □ 2 silicone 30 0.8 12 ◯ □ particles (spherical) 10 25 ◯ □ 20012 ◯ □ 25 ◯ □ 12 ◯ □ 25 ◯ □ 3 Crosslinked 10 0.8 6 ◯ □ polyacrylate(perfectly spherical) 10 10 ◯ □ ester particles 200 6 ◯ □ 10 ◯ □ 6 ◯ □10 ◯ □ 4 Crosslinked 30 0.8 10 ◯ □ polyacrylate (perfectly spherical) 1025 ◯ □ ester particles 200 10 ◯ □ 25 ◯ □ 10 ◯ □ 25 ◯ □ 5 Polyurethane 90.8 5 ◯ □ particles (perfectly spherical) 10 8 ◯ □ 200 5 ◯ □ 8 ◯ □ 5 ◯ □8 ◯ □ 6 silicone mixture of examples 1 0.8 6 ◯ □ particles, and 3 with aratio of 10 10 ◯ □ crosslinked 1:1 200 6 ◯ □ polyacrylate 10 ◯ □ ester 6◯ □ particles 10 ◯ □

EXAMPLE 7

Mixed together were 100 parts by weight of acrylic-based photo-curableresin, 20 parts by weight of silicone particles (with a rubber hardnessof 30) having 30 μm in average particle diameter, 5 parts by weight of areactive diluent, 5 parts by weight of methyl ethyl ketone, 2 parts byweight of a photo initiator and 1 part by weight of a leveling agent,and the mixture was extruded through a die to form a film aftertransfer. Prepared were films of 50 μM and 100 μm in thickness. Thefilms were irradiated with light and cured to obtain desired reflectionfilms containing elastic particles. Inspection was conducted on levelsof damage and light spots on the reflection films in a similar manner tothat conducted in Examples 1 to 6. The level of damage on each of thelight guiding plates was observed with a microscope to thereby enable arecess on the slightest scale to be recognized without any practicalproblem being posed, while the light spots were at a level of norecognition.

COMPARATIVE EXAMPLES 1 TO 6

A resin layers containing particles on the reflection surface of thereflection film was formed in a similar manner to that employed inExamples 1 to 6. The particles were all non-elastic. In Table 2, thereare shown specifications of particles and levels of damage and levels oflight spots on the light guiding plates caused by the reflection filmobtained. Note that a column of added amounts of Table 2 shows part orparts by weight of elastic particles added into 100 parts by weight of abinder resin and symbols in columns of damage and light spots on thelight guiding plates are evaluated at three ratings, in which a mark □is a level at which neither damage nor a light spot is recognized on adisplay when a light source is turned on, a mark ∘ is a level at which arecess on the slightest scale can be recognized without any practicalproblem being posed and a symbol x is a problematical level at whichdamage or light spots can be recognized. TABLE 2 added amounts damage onComparative average diameter (parts by film thickness light guidinglight Example. elastic particles (μm) weight) (μm) plate spots 1 acrylicresin 8 0.8 6-7 x x particles (perfectly 10 6-7 x x spherical) 200 6-7 xx 2 acrylic resin 15 0.8 12 x x particles (perfectly 10 12 x xspherical) 200 12 x x 3 silica particles 3 0.8 2 x x (perfectly 10 2 x xspherical) 200 2 x x 4 Polyethylene 8 0.8 7 x x particles (perfectly 107 x x spherical) 200 7 x x 5 Benzoguanamine 15 0.8 13 x x formaldehyde(spherical) 10 13 x x condensate 200 13 x x particles 6 calciumcarbonate 2-3 0.8 3 x x particles 10 3 x x 200 3 x x

COMPARATIVE EXAMPLES 7 TO 8

In Comparative Example 7, a reflection film obtained from a whitepolyester-based resin containing calcium carbonate as an inorganicadditive was used to construct the backlight optical system, while inComparative Example 8, a reflection film obtained from a whitepolyolefin-based resin containing barium sulfate as an inorganicadditive similarly was used to construct the backlight optical systemand inspection was conducted on levels of damage and levels of lightspots on light guiding plates. In both Comparative Examples 7 and 8,damages or light spots in both cases were able to be recognized andpractically problematic.

As it is clear from Comparative Examples 7 and 8, in cases wherebacklight optical systems were constructed with the reflection filmsobtained from a white polyester-based resin containing an inorganicadditive and a white polyeolefin-based resin containing an inorganicadditive, damages and light spots on both light guiding plates were ableto be recognized and practically problematic. In cases of reflectionfilms provided with resin layers containing ordinary particles with noelasticity as shown in Comparative Examples 1 to 6, damages or lightspots on all light guiding plates were able to be visually recognizedand were practically problematic. In all of Comparative Examples 1 to 8,clear dot-like damages were recognized on the light guiding plates, andthe light guiding plates were used in the backlight optical systems,with the results of qualities of displays being at a low level withlight spots being observed thereon.

On the other hand, in Examples 1 to 7, backlight optical systems wereconstructed using reflection films on which resin layers containingelastic particles, or alternatively, reflection films containing elasticparticles therein, with the result that either damage or a light spotwas hard to be recognized on all of the light guiding plates, andaccordingly, with light spots being hard to be recognized on all of thelight guiding plates, qualities of displays using the backlight opticalsystems were each at a level of no practical problem.

INDUSTRIAL APPLICABILITY

By constructing a backlight optical system with a reflection film havinga resin layer containing particles with elasticity on a surface, incontact with a light guiding plate, thereof, it is possible to preventthe light guiding plate, especially, the light guiding plate made of apolyolefin-based resin, from being damaged. As a result, a high qualitydisplay free of light spots can be constructed. The resin layercontaining elastic particles can be provided on a reflection film in asimple process at a low cost.

Reduction in weight and downsizing have been difficult in case of anacrylic resin plate which were conventionally employed as a lightguiding plate. On the other hand, the invention realizes reduction inweight and downsizing of a liquid crystal display and provides areflection film free of light spots.

1. A reflection film used in a backlight optical system consisiting of a light source, a light guiding plate and a reflection film, wherein a resin layer containing particles with elasticity is provided on a surface, in contact with the light guiding plate, of the reflection film.
 2. A reflection film used in a backlight optical system consisting of a light source, a light guiding plate and a reflection film, wherein the film contains particles with elasticity therein.
 3. The reflection film according to claim 1 or 2, wherein the light guiding plate is made of a polyolefin-based resin.
 4. The reflection film according to any of claims 1 to 3, wherein the particles with elasticity each have a rubber hardness of 50 or less.
 5. The reflection film according to any of claims 1 to 4, wherein the particles with elasticity each are made of at least one kind selected from a group consisting of silicone, crosslinked polyacrylate ester and polyurethane.
 6. The reflection film according to any of claims 1 to 5, wherein the particles with elasticity each are spherical.
 7. The reflection film according to any of claims 1 to 6, wherein diameters of the particles with elasticity each are in the range of from 1 to 60 μm.
 8. The reflection film according to any of claims 1 to 7, wherein diameters of the particles with elasticity each are in the range of from 1 to 40 μm.
 9. The reflection film according to any of claims 1 to 8, wherein the average diameter of the particles with elasticity is in the range of from 5 to 20 μm.
 10. The reflection film according to any of claims 1 to 9, wherein the film is made of a polyester-based resin or a polyolefin-based resin with microvoids therein and the film contains wherein an inorganic filler.
 11. The reflection film according to any of claims 1 to 10, wherein the film is made of a white polyester-based resin film containing an inorganic filler.
 12. The reflection film according to any of claims 1 to 10, wherein a vapor deposited layer made of silver or aluminum is provided on the film. 